Final answer:
When an object is in equilibrium, its acceleration is always zero, indicating it is either at rest or moving at constant velocity with no net force acting upon it. Equilibrium does not preclude an object from having momentum or energy, as these quantities are not inherently zero in an equilibrium state.
Step-by-step explanation:
When an object is in equilibrium, the value that is always zero is its acceleration. This is because an object in equilibrium is not undergoing any change in velocity; it is either at rest or moving at a constant speed in a straight line. In such cases, according to Newton's first law of motion, the net force acting on the object is zero, and thus, there is no acceleration. It's important to note that being in equilibrium does not necessarily mean that the object's velocity is zero, as an object can be in a state of dynamic equilibrium while moving at a constant velocity.
When discussing changes in momentum, the rate of change of momentum is equivalent to the net force acting on the object. Therefore, if the momentum of an object increases over time, a nonzero net force must be acting on it. In terms of rotational motion, an object remains in rotational equilibrium when the net torque acting on it is zero and there is no change in its angular momentum.
In summary, the concept of equilibrium in physics involves conditions where acceleration is zero, whether the object is static (at rest) or in dynamic equilibrium (moving with constant velocity). The momentum of a body changes only when a net force is applied, and the rate of change of this momentum is directly proportional to the net force.